Table 1.

Igf1 expression in mice

	Genotype	cDNA expression status
		Allele 1	Allele 2
2X	Igf1Δflox/Δflox	ON	ON
1X	Igf1Δflox/	ON	OFF
1XL	Igf1Δflox/flox; AT-cre	ON	OFF→ON
LIP	Igf1flox/flox; AT-cre	OFF→ON	OFF→ON
Null	Igf1flox/floxflox/flox	OFF	OFF

Exon 4 of both alleles in all of these mice has been replaced with a neo cassette. An Igf1 cDNA connected to an upstream floxed block has been transcriptionally activated by Hs-cre and is ubiquitously expressed both embryonically and postnatally in 2X mice (both alleles) and in 1X and 1XL mice (1 allele). The second allele of 1XL mice and both alleles of LIP mice are activated postnatally by AT-cre only in the liver.